The history of particle physics

From the ancient Greeks to the present day, what do we know about the building blocks of the Universe?
12 October 2021

Interview with 

Frank Close, University of Oxford


First up, let’s go back in time to understand where the fascination with particles began. Cameron Voisey heard from the University of Oxford's Frank Close... 

Frank - The quest to understand what matter is made of goes back to the ancient Greeks who postulated that everything was made of atoms.

Cameron - That's Frank Close, Emeritus Professor of Physics at Oxford University,

Frank - And a century ago, that's what we thought the answer was -  that the atoms of the elements, hydrogen, helium through to uranium, are what made everything. Since then, we know that atoms are not the smallest thing. They're made of smaller pieces -  there's a central nucleus made of protons and neutrons around which circle tiny electrons. And so electrons, protons and neutrons are the basic constituents of all atoms. But in the last 50 years, we know that that isn't the final story (it's like peeling off layers of an onion), that although electrons appear to be basic seed to everything the protons and neutrons are made of smaller bits, little particles called quarks. So today the basic pieces from which all matter that we know of are made are electrons and quarks and particles of that sort.

Cameron - But is the end of the story? How many particles are there?

Frank - To make the world that we are familiar with, three are sufficient, but there are many more than that in practice. For some reason, nature wasn't satisfied with the basic set that make you and me, the stable things like the electron and the particular quarks that make protons and neutrons. It made two heavier versions involving things called muons, which are like heavy electrons, charmed and strange quarks, which are heavier than the ones that make protons and neutrons and top and bottom quarks, which are even heavier still. In addition to those, there are some ghostly, electrically neutral particles called neutrinos .

Cameron - The basic idea behind particle physics is to breakup objects until they can be broken up no more and then to study these things. The way these objects behave has far-reaching consequences, not only do they influence how things are constructed on the smallest scales, but on the bigger scales too. That's because the way particles behave in the early universe is responsible for how the universe looks today. That's one reason why it's so important we understand them. By piecing together lots of experimental information over the past half century physicists have been able to construct a theory of how these tiny little objects behave. It's called the Standard Model of particle physics. And it's one of scientist's most accurate and well-tested theories. Following a 50-year search, the final piece of this jigsaw was discovered in 2012 -  the Higgs boson. The existence of this particle is a crucial part of the Standard Model and its discovery signals that the way particle physicists have seen the world up to now makes sense. So does that mean Frank's out of a job? Well, no.

Frank - We know a huge amount about 5% of everything. The reason being that the stuff that makes you and me is but flotsam on a sea of what's called Dark Matter. We know from the behavior of the galaxies, the way that the galaxies tug on one another, there's more gravitational pull than we can account for, than what we see through our telescopes . And so this dark stuff is called dark matter. There seems to be no real doubt that it exists, but what it consists of at the particle level, we don't know.

Cameron - And worse still, dark matter is just one member of a set of conundrums that keep particle physicists up at night. These problems range from the nuanced to the downright troubling. For example, according to the Standard Model, the universe should not just be made up of the matter that we're all familiar with. There should be an almost equal part of stuff called anti-matter, which actually annihilates matter when it meets. Where is this stuff? Well, while it does exist, there should be more of it than we're currently detecting. It's just one more sign that there must be something out there that we don't understand.

Frank - So there are many unknowns. It's a bit like trying to get to the end of the rainbow. You don't know how many things are on the way, but as you try to get there, you pass a lot of things that you didn't know were there before. The prediction of the Higgs Boson was by analogy telling us how high the mountains were that we had to climb. So for the last 30 odd years, we have had an idea of how high the mountain range is and that if we could get to the peak, we would find the bows on them. Indeed, 10 years ago, that is what happened. So we have conquered that mountain range, and we now see before us the plains stretching out and somewhere out there is the next mountain range, which will contain the answers to the questions that we're now trying to really understand as to why is it that the Standard Model is as it is. The problem is that we don't know how far we have to travel to find that mountain range. We don't know how high those mountains are at the moment. We know nothing about them, other than a certainty that they are out there somewhere, but how far away we don't know.

Cameron - But particle physics is in a curious position. There's clear evidence that something lies beyond our current understanding. And yet, so far, the standard model is proven robust. It's just too good. After almost 10 years of experiments pumping out data since the discovery of the Higgs Boson, no new fundamental particles have been discovered.

Frank - It isn't complete, but there is nothing in our model that we have yet found which is "wrong". And in fact, that is part of the problem -  that we know that this model cannot be the final answer, because there are many things about it we don't understand. So there is something beyond the Standard Model. The answers are clearly out there somewhere, but how far we have to travel to find them we just don't yet know.

Cameron - Making progress seems like a pretty daunting task. So how does anyone know where to look for answers?

Frank - So you can do strategic things by designing your experiments cleverly, to look for certain things that theorists think might be there, but serendipity happens that it's quite possible that discoveries come out of the blue and that's one of the excitements of science. We don't know what it will be, but if there is something out there, we will certainly have a new way of looking at the universe. And hopefully that something will be the explanation of why things are as they are.


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